Caustic cleaning compositions are widely used in a variety of applications for removing a wide variety of organic and inorganic deposits. Sodium hydroxide, or caustic soda, is highly reactive with such deposits.
Caustic cleaning compositions have a freezing point that is too high for many applications. A typical caustic cleaning composition has a sodium hydroxide concentration ranging from about 25 to about 49% by weight. At such concentrations, the caustic cleaning composition can have a freezing point ranging from about 50.degree. F. to about 88.degree. F. Although it is known to add salts to the caustic cleaning composition, the depression in the freezing point of the composition from such additives is commonly relatively small (i.e., less than about 10.degree. F.)
There is a need for a caustic cleaning composition having a relatively low freezing point.
SUMMARY OF THE INVENTION
These and other needs are addressed by the caustic cleaning composition of the present invention and the method for its manufacture. The caustic cleaning composition comprises (a) at least about 35% by weight sodium hydroxide and (b) an organic salt derived from the reaction of gluconic acid with sodium hydroxide. Most preferably, the composition comprises from about 2 to about 5% by weight of the organic salt.
The organic salt is believed to behave like a surfactant in the composition and significantly lower the freezing point of the composition. For a caustic cleaning solution having from about 35 to about 50% by weight sodium hydroxide, the freezing point of the composition is preferably no more than about 40.degree. F. and most preferably ranges from about 24 to about 36.degree. F.
While not wishing to be bound by any theory, it is believed that the salt can be a cyclic ring compound, such as a lactone and/or a straight chain compound having one or more attached hydrocarbyls containing 12 to 36 carbon atoms. The salt is believed to form in response to the latent heat of reaction between sodium hydroxide and water (which is added to the sodium hydroxide before and/or after the contacting step) and/or between sodium hydroxide and gluconic acid. The derivation of the salt in the aqueous sodium hydroxide solution from the reaction of gluconic acid and sodium hydroxide is believed to enhance the solubility of the salt in the solution compared to the addition of the salt directly to the solution.
The caustic cleaning composition can include other additives to further enhance the composition's properties. By way of example, the composition can include a hydroxy acid salt, namely a gluconate with a heptagluconate being most preferred. The gluconate acts as an inhibitor for oxalate formation during cleaning of kettles and other equipment used in the brewing of beer. The composition preferably includes from about 0.1 to about 3%. by weight of the gluconate. The composition can include an inorganic salt, preferably derived potassium hydroxide, to provide a further reduction in the freezing point of the composition. The inorganic salt is preferably added in an amount near its solubility limit in the composition. Preferably, the inorganic salt has a concentration ranging from about 1 to about 8% by weight of the composition.
The process for producing the caustic cleaning composition is an important aspect of the present invention. The process includes contacting the gluconic acid with the aqueous sodium hydroxide solution to form an organic caustic solution. It is important that the gluconic acid be added to a vessel containing the sodium hydroxide. Surprisingly, the same freezing point depression is not realized when the sodium hydroxide is added to a vessel containing the gluconic acid. During the contacting step, the sodium hydroxide solution preferably has a temperature ranging from about 72 to about 122.degree. F. The preferred molar ratio of the gluconic acid to the sodium hydroxide ranges from about 1 to about 100. The molar ratio typically produces a concentration of the organic salt in the caustic solution ranging from about 1.5 to about 9% by weight. The strongly exothermic reaction between the gluconic acid and the sodium hydroxide is believed to produce an uneven temperature distribution in the solution with the highest temperature(s) being at the point(s) of addition of the gluconic acid.
After the contacting step, first the inorganic salt and second the gluconate are added to the solution.